[meta-xilinx] meta-xilinx preparation for dora branch

[Sipke Vriend]

Hi,

We have pushed some changes to meta-xilinx in preparation for creating
a dora branch in the near future.
The 3.8 kernel recipes now use the linux-xlnx.git xlnx_3.8 branch.
The 3.10 kernel recipe source revision now matches xilinx-14.7 tag.
There have been a few other minor changes to recipes.

Regards
Sipke